Photoconductive device having a target including a selenium blocking layer



Oct. 8, 1968 DRESNER 3,405,298

PHOTOCONDUCTIVE} DEVICE HAVING A TARGET INCLUDING A SELENIUM BLOCKING LAYER Filed March 4, 1965 EEM United States Patent 3,405,298 PHOTOCONDUCTIVE DEVICE HAVING A TARGET INCLUDING A SELENIUM BLOCKING LAYER Joseph Dresner, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Mar. 4, 1965, Ser. No. 437,252 9 Claims. ('Cl. 313-65) ABSTRACT OF THE DISCLOSURE A photoconductive target comprises a conducting signal electrode, a layer of selenium alloy containing selenium, tellurium and arsenic or antimony, and an intermediate layer of substantially pure selenium having a thickness of from 100 Angstroms to 600 Angstroms.

This invention relates to photoconductive devices and particularly to a photoconductive device having a composite photoconductive structure of improved electrical characteristics.

One type of photoconductive device in which the invention is particularly useful is the vidicon pickup tube. Vidicon pickup tubes generally comprise an evacuated glass bulb within which is mounted an electron gun of any suitable construction for developing a reflectible beam of electrons. These electrons are focused and deflected in a desired scanning pattern over a photoconductive target. The photoconductive target has the property of co-acting with the scanning electron beam and with the light impinging on each successive elemental area of the target. The impinging light causes the elemental areas of the target to change their conductivity proportionately to the light intensity and this change in conductivity produces signal currents under control of the scanning beam.

The photoconductive target is usually supported on a light-transparent backing which may be the end wall or faceplate of the blass bulb or tube envelope. The target usually consists of a highly conductive light-transparent layer or signal plate, which is applied to the inner surface of the faceplate facing the electron gun, and a layer of photoconductive material applied over the transparent conductive layer.

One photoconductive material that can be used is vitreous selenium or a suitable alloy thereof. Pure selenium exhibits a sensitivity to blue light primarily while an alloy including selenium, tellurium, and arsenic or antimony is characterized by a panchromatic spectral response.

In many applications, a panchromatic spectral response is desirable. However, the use of a selenium alloy of the type described in the photoconductor structure of a photoconductive device gives rise to a serious problem. This problem concerns an objectionably large dark current of the device. A relatively large dark current appreciably reduces usefulness of a device in which a selenium alloy photoconductor is used. The problem with respect to dark current is even more severe when the selenium alloy is deposited under high vacuum conditions such as 10 torr or lower, in order to obtain increased sensitivity and speed of response.

Accordingly, it is an object of the invention to provide a selenium-containing photoconductive structure of improved electrical characteristics.

A further object is to provide a photoconductive device incorporating photoconductor structure contributing to a reduction in the dark current of the device.

Another object is to provide a selenium alloy photoconductor of relatively high sensitivity and speed of response, that is free from excessive dark current.

A further object is to provide a photoconductor struc- 3,405,298 Patented Oct. 8, 1968 ture containing a selenium alloy for panchromatic spectral response in association with a conductive layer, in which the alloy does not make an ohmic contact with the conductive layer.

The foregoing objects are realized in a photoconductive structure which includes a layer of conductive material, a layer of selenium alloy having a panchromatic spectral response, and an intermediate blocking layer of pure selenium. The pure selenium has a larger bandgap than the selenium alloy, so that the presence of the pure selenium appreciably increases the contact resistance between the conductive layer and the selenium alloy layer. The in-, crease thus effected in contact resistance drastically reduces the dark current of a device in which the photoconductive structure of the invention is used, to a value where it does not significantly impair the usefulness of the device.

The sole figure of the drawing is a fragmentary view partly broken away of a vidicon tube 10 in which the photoconductor structure or target 12 incorporates the present invention. The tube 10 is conventional except for the target 12. The tube 10 comprises an elongated glass envelope 14 closed at one end thereof by a transparent glass faceplate 16. The faceplate is sealed across the end referred to by means of an indium ring 18 and a clamping ring 20. The target 12 is positioned on the inner surface of the faceplate 16. Closely spaced from the target 12 is a mesh screen 22 mounted across one end of a tubular focusing electrode 24. In the other end portion (not shown) of the envelope 14 is positioned an electron gun for providing an electron beam. The electron beam is scanned across the target 12 by suitable means such as electromagnetic coils (not shown) disposed outside of the envelope 14.

The target 12 is a multi-layer structure. It includes a relatively thin layer or film 26 of an electrically conducting material such as tin oxide, in contact with the faceplate 16. The conducting layer 26 is light-transparent and serves as a signal electrode. The layer 26 may have a voltage of +30 volts impressed thereon in operation. The target also includes a layer 28 of selenium alloy consisting by weight of from 15% to 27% tellurium, 0.5% to 5% arsenic or antimony, and the balance selenium. Between the selenium alloy layer 28 and the signal plate 26 is disposed a blocking layer of substantially pure selenium 30. The function of the blocking layer 30 is to shield electrically the selenium alloy layer 28 from the electrically conducting film 26.

The selenium alloy layer 28 and the substantially pure selenium blocking layer 30 may be thermally stabilized in the vitreous state, by a stabilizing undercoating 32 and by a stabilizing overcoating 34. Such stabilization is desirable for preserving the selenium alloy layer 28 and the selenium blocking layer 30 in the vitreous state. The desired stabilization is eflected by mechanical restraint by the undercoating 32 and the overcoating 34 of crystal enlargement at the surfaces of the layers 28, 30. The undercoating 32 may be made of a material such as gold, silver, copper, rhodium, palladium, germanium and germanium oxide, antimony trisulfide, gallium, gallium oxide, iridium or antimony. The preferred material is gold. The overcoating 34 may be made of germanium, germanium oxide, antimony trisulfide or antimony trioxide.

When the pure selenium and selenium alloy layers are not thermally stabilized, the pure selenium layer 30 may be deposited directly on the signal plate 26, and the outer stabilizing layer 34 may be omitted. If stabilizing means other than that described in the foregoing should be used, it may be substituted for one or both of the layers 32 and 34, or otherwise incorporated in the photoconductor structure. In any event, it is important that the selenium alloy layer 28 'be shielded electrically from an adjacent conducting member, by the layer 30 of pure vitreous selenium.

Such shielding substantially eliminates ohmic contacts between the conducting member and the selenium alloy during operation of the device. Such substantial elimination of ohmic contacts results in an appreciable reduction in dark current of the device in which these layers are used.

Effective temperature. stabilization of the composite photocondu'ctor structure comprising the selenium alloy layer 28 and the pure selenium layer 30 is achieved by one or both of the stabilizing layers 32, 34. The presence of an interface between the selenium alloy layer 28 and the pure selenium layer 30 does not result in loss of temperature stabilization. Freedom from such loss in temperature stabilization at the interface is believed to be due to the effective mechanical restraint to crystal enlargement that the two interface-forming surfaces mutually provide.

In the example of a photoconductive target structure 12, the thickness of each of the several layers of the structure is of a preferred value for good results.

Thus, the conducting layer or signal plate 26 should be thin enough to be light-transparent and sufliciently thick to have required lateral conductance for service as the signal electrode of the tube. These requirements are satisfied by a thickness of about one micron forthe case of tin oxide. Although the stabilizing underlayer 32 may be omitted, its use is desirable in contributing to a relatively long thermally stabilized life of the pure vitreous selenium blocking layer 30 and the vitreous selenium alloy layer 28. If incorporated in a photoconductive target structure, it is desirable that the stabilizing underlayer 'be restricted to a thickness range from about 6 to about 30 Angstroms for good results. The ure selenium layer 30 should have a suificient thickness for service as a blocking layer. The thickness of the layer 30 should not be less than about 100 Angstroms nor more than about 1800 Angstroms for satisfactory results. However, experiments have indicated that the optimum thickness range of the layer 30 is from about 100 to about 600 Angstroms. The selenium alloy layer 28 should have a thickness within the range of from about 3 to about 10 microns. The stabilizing overlayer 34, it used, should have a thickness of from about 10 to about 60 Angstroms.

In the target structure described, the blocking layer 30 of selenium is effective to prevent ohmic contacts between a conductive substrate, such as the stabilizing layer 32, and the seleniumalloy layer 28, because of the ap preciably larger bandgap of .pure selenium. Such larger bandgap of the selenium causes it to serve effectively as an electrical insulator in the absence of light energization, to thereby substantially eliminate one cause for dark current in a device in which the target structure 12 is used.

I claim:

1. A photoconductive device comprising:

(a) a glass envelope (b) an electrically conductive layer on a portion of said envelope,

(c) a selenium alloy layer over said electrically conductive layer; having a panchromatic spectral response, and

(d) a blocking layer of substantially pure vitreous selenium between said conductive layer and said selenium alloy layer, for reducing ohmic contact between said conductive layer and said selenium alloy layer.

2. A photoconductive device comprising:

(a) an envelope having a transparent faceplate,

(*b) an electrically conductive layer on said faceplate,

(c) a vitreous selenium alloy layer over said electrically conductive layer having a relatively small band- (d) a blocking layer of substantially pure vitreous selenium having a relatively large bandgap, between said conductive layer and said selenium alloy layer, for eliminating ohmic contacts between said conducti-ve layer and said selenium alloy layer, and

(e) means engaging at least one of said vitreous alloy and said blocking layers for temperature stabilizing the vitreous state thereof.

3. A photoconductive device comprising:

(a) an envelope having a transparent faceplace,

, 4. A photoconductive device comprising:

(a) an electrically conductive layer,

(b) a vitreous selenium alloy layer having a panchromatic spectral response, said selenium layer having a thickness of from about 3 microns to about 10 microns, and

(c) a blocking layer of substantially pure vitreous selenium between said conductive layer and said selenium alloy layer, said blocking layer having a thickness of from about to 1800 Angstroms.

5. A photoconductive device comprising:

(a) an insulating substrate,

(b) a first layer of electrically-conductive material on said substrate,

(0) a second layer of substantially pure vitreous selenium on said first layer,

(d) a third layer of a vitreous alloy consisting by weight of from about 15% to about 27% tellurium, from about 0.5% to about 5% of a material selected from the group consisting of arsenic and antimony, and the balance selenium, and

(e) means engaging said second and third layers for thermally stabilizing the vitreous character of said second and third layers.

6. A photoconductive device comprising:

(a) an insulating substrate,

(b) a first layer of electrically-conductive material on said substrate,

(c) a second layer of substantially pure vitreous selenium on said first layer, and

(d) a third layer on said second layer consisting of a vitreous alloy consisting by weight of from about 15% to about 27% tellumium, from about 0.5% to about 5% of a material selected from the group consisting of arsenic and antimony, and the balance selenium.

7. A photoconductive device comprising:

(a) an insulating substrate,

(b) a first layer of electrically-conductive material on said substrate,

(c) a second layer of substantially pure vitreous selenium on said first layer, said second layer having a thickness of from about 100 Angstroms to about 1800 Angstroms, and I (d) a third layer of a vitreous alloy consisting by weight of from about 15% to about 27% tellumium, from about 0.5 to about 5% of a material selected from the group consisting of arsenic and antimony, and the balance selenium, said third layer having a thickness of from about 3 to about 10 microns.

8. A photoconductive device comprising:

(a) an insulating substrate,

(b) a first layer of a first electrically-conductive mate- )(d) a third .layer of substantially pure vitreous selenium on said second layer, and

(e) a fourth layer of avitreous alloy consisting by 5 Weight of from about 15% to about 27% of a material selected from the group consisting of tellurium, from about 0.5% to about 5% arsenic and antimony, and the balance selenium.

9. A photoconductive device comprising:

(a) an insulating substrate,

(b) a first layer of a first electrically-conductive material on said substrate,

(c) a second layer of a second electrically-conductive material on said first layer,

(d) a third layer of substantially pure vitreous selenium on said second layer,

(e) a fourth layer on said third layer of a vitreous alloy consisting by Weight of from about 15% to about 27% tellurium, from about 0.5% to about 5% of a material selected from the group consisting of arsenic and antimony, and the balance selenium, and

(f) a fifth layer on said fourth layer, of a material serving to thermally stabilize said fourth layer in its vitreous state.

References Cited UNITED STATES PATENTS JAMES D. KALLAM, Primary Examiner. 

